1. Field of the Invention
The present invention relates to a light emitting device using a light emitting element from which florescence or phosphorescence is obtained by applying an electric field to an element having a film containing organic compound (hereinafter referred to as organic compound layer) between a pair of electrodes, and to a method of manufacturing the same. Note that the term light emitting device in this specification refers to an image display device, a light emitting device or a light source. Also included in the definition of the light emitting device are: a module in which a connector, such as an FPC (flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package), is attached to an organic light emitting device; a module in which a printed wiring board is provided on the tip of a TAB tape or a TCP; and a module in which an IC (integrated circuit) is mounted directly to an organic light emitting device by the COG (chip on glass) method.
2. Description of the Related Art
In recent years, a technique for forming a TFT on a substrate has progressed substantially, and application and development of active matrix type display devices is advancing. Especially, since a TFT using a polysilicon film has a field-effect mobility (also called mobility) higher than that of a TFT using a conventional amorphous silicon film, a high speed operation is possible.
Such active matrix display devices are attracting attention because, various merits such as reduction of manufacturing cost, miniaturization of a display device, improvement of a yield, and reduction of a throughput can be obtained by forming various circuits and elements on the same substrate.
Among them, in a light emitting device in which light emitting elements made from an anode, an organic compound and a cathode are arranged in a matrix form (hereinafter referred to as active matrix light emitting device), a switching element formed of a TFT (hereinafter referred to as switching TFT) is provided for each pixel, and a driving element for controlling current (hereinafter referred to as current control TFT) is operated by the switching TFT, thereby making the light emitting elements emit light.
Note that a light emitting element is an element that emits light when an electric field is applied. Light emission mechanism thereof is said to be as follows. A voltage is applied to an organic compound film sandwiched between electrodes to cause recombination of electrons injected from the cathode and holes injected from the anode in the organic compound film, and when the resultingly excited molecule (hereinafter referred to as molecular exciton) returns to base state, it releases energy in the form of light emission.
In such a light emitting element, its organic compound layer is usually formed of a thin film having a thickness of less than 1 μm. In addition, the light emitting element does not need a backlight used in conventional liquid crystal displays because it is a self-luminous element so that the organic compound layer itself emits light. The light emitting element is therefore useful in manufacturing very thin and light-weight devices, which is a great advantage.
With those features, including thinness, light-weightedness, quick response, and direct current low voltage driving, light emitting elements are attracting attention as the next-generation of flat panel display elements. In addition, since the light emitting elements are self-luminous and have a wide viewing angle, relatively satisfactory visibility is provided and they are considered as effective especially when used for display screens of electric appliance. However, the following points were problems.
Usually, at least one or two TFTs are given for each pixel on the substrate. Further, through selection of TFT, a source signal line and a gate signal line are formed in order to turn the device ON. Further, in order to insulate the TFT from light emitting elements, an interlayer insulating film which consists of insulated materials, such as silicon oxide and silicon nitride is formed on the TFT. Then, as the TFT thickness is 0.2 to 1 μm and uneven, this had to be avoided while pixel electrodes were formed. Note that in this case, since the region in which the pixel electrodes are formed is substantially made smaller, there was a problem on that the aperture ratio of a pixel portion fell.
On the contrary, JP-10-189252A discloses the following technique. There are used a polyamide coating layer formed by spin coating and a layer formed by an etch-back method after silica is subjected to polymer coating, thereby forming an interlayer insulating film on the TFT to perform leveling. Further, the light emitting element is formed thereon, thereby improving the aperture ratio of the light emitting device.
In such light emitting devices, electric connection with a TFT formed on a substrate is made through an interlayer insulating film. The interlayer insulating film is formed from an inorganic material containing silicon, such as silicon oxide, silicon nitride, and silicon oxynitride, or from an organic material such as polyimide, polyamide, acrylic, and other organic resins.
Inorganic materials have a characteristic that does not allow moisture and oxygen to transmit but have a defect of being cracked when they are formed into thick films.
In contrast, organic materials can be formed into thick films and the surfaces of the films are fairly level. Accordingly, a film formed from an organic material is suitable as a film to level the surface above a TFT. However, organic materials also have disadvantages such as being transmissive of oxygen and transmissive or absorptive of moisture.
An organic compound layer constituting a light emitting element is very weak against oxygen and moisture and is easily degraded. Oxygen and moisture cause degradation of a light emitting element and accordingly cause dark spot or other degradation of a light emitting device.
When a second interlayer insulating film formed of an organic material and a third interlayer insulating film formed of an inorganic material are layered, there is a technical difficulty in patterning the laminate. Specifically, the third interlayer insulating film peels off (peeling) at an edge of a contact hole where a first interlayer insulating film and the second interlayer insulating film overlap and are exposed in section. The contact hole is formed in the laminate in order to form a wire for connecting the TFT with an electrode of the light emitting element.